Visual and quantitative characterization of ironwood tree (Casuarina equisetifolia) decline on Guam
Z. MERSHA (1), R. L. Schlub (1), P. O. Spaine (2), J. A. Smith (3), S. C. Nelson (4)
(1) UOG Cooperative Extension, University of Guam, Mangilao, GUAM; (2) USDA Forest Service, Athens, GA, U.S.A.; (3) University of Florida’s Institute of Food and Agricultural Sciences, Gainesville, FL, U.S.A.; (4) Cooperative Extension Service, University of Hawaii At Manoa, Hilo, HI, U.S.A.
Phytopathology 100:S82

To assess the level of ironwood tree decline on Guam, photographs of 44 randomly selected trees with varying levels of decline were categorized into small (CBH ≤ 100 cm) or large (CBH >100 cm) based on their circumference at breast height (CBH) and visually catalogued into a five-scale decline severity (DS) rating. On subsequent surveys, trees with different DS ratings were characterized visually for branch thinning and quantitatively for branchlet (“needle”) biomass. As DS increased from 0 (healthy) to 4 (nearly dead) branch thinning, progressively increased from 0 to 95.0% and 0 to 92.5% for small and large trees, respectively. There was no significant difference between branchlet biomass for DS 0 and DS 1 nor between DS 2 and DS 3 trees. The greatest branchlet weight loss, at 95.3%, occurred to DS 4 trees. Internal symptoms included various patterns of discolorations in trunks and a white soft-rot in roots. Discoloration was consistently traced into branches through cross-sectioning at the branch trunk interface. In branches the presence of discoloration was only 100% reliable for DS 3 and 4 trees. External symptoms starts at the top of trees and progresses downward; whereas, internal discoloration starts at the tree’s base and diminishes acropetally This gradient of discoloration was well described by the exponential decay function (P = 0.0001) with R² values of 0.85 and 0.73 for small and large trees, respectively. Ironwood tree decline has expanded into areas such as Cocos Island, which were virtually decline free a year earlier.